Asymmetrical bus keeper

ABSTRACT

Various embodiments are described herein for an asymmetrical bus keeper circuit that provides asymmetrical drive towards one logic level. The asymmetrical bus keeper circuit comprises a first inverter stage having an input node and an output node, an asymmetrical inverter stage having an input node and an output node and a feedback stage with an input node and an output node. The input node of the asymmetrical inverter stage is connected to the output node of the first inverter stage. The input node of the feedback stage connected to the output node of the asymmetrical inverter stage and the output node of the feedback stage connected to the input node of the first inverter stage. The asymmetrical stage provides asymmetrical drive towards one logic level.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/398,929, filed Feb. 17, 2012, which is a continuation-in-part of U.S.patent application Ser. No. 13/272,653, filed Oct. 13, 2011. U.S. patentapplication Ser. No. 13/272,653 has been abandoned. The entire contentsof U.S. patent application Ser. No. 13/398,929 and U.S. patentapplication Ser. No. 13/272,653 are hereby incorporated by reference.

FIELD

The various embodiments described herein generally relate to a buskeeper circuit with an asymmetrical configuration for providing anasymmetrical drive to only one logic level.

BACKGROUND

In computer systems or in some devices that perform computer functionsor behave as computers, several circuits or devices are typicallyconnected with one another via a communication bus so that they cancommunicate with each other. If the devices do not have to provide anyinformation to the bus (i.e. the devices are inactive), the devices gointo a high-impedance state. In this case, a pull-up resistor weaklypulls the voltage on the bus to the high logic level. When one of thedevices becomes active, it will override the logic level that ismaintained by the pull-up resistor in order to communicate on the bus.

In the case of a tri-state bus, there are three scenarios for the outputof the devices that are connected to the bus: a logic low level, a logichigh level and a floating level (i.e. a tri-state mode where the devicesare not active). In contrast to the previous pull-up resistor example, abus keeper circuit (also known as a bus-holder circuit) is connected tothe tri-state bus to weakly hold the last logic value on the bus. Thisis done to prevent a floating voltage level on the bus since, in thiscase, the gates of both input transistors of a device that is connectedto the tri-state bus can turn on which essentially provides ashort-circuit connection between the power supply and ground which candamage the gates. Conventional bus keeper circuits include a first CMOSinverter connected in series to a second CMOS inverter and a resistorconnected in series to the second inverter. The transistors used in eachinverter have a similar amount of drive and therefore the inverters aresymmetrical. The first inverter and the resistor are connected to thetri-state bus such that the bus keeper circuit is connected in parallelwith the tri-state bus. The resistor drives the bus weakly to either thelow logic level or the high logic level (i.e. whichever was the lastlogic value on the bus) and the other circuits that are connected to thetri-state bus can override the value of the bus when they are not in thetri-state mode.

The conventional bus keeper circuit is suitable for use withcommunication buses that have symmetrical impedance drive. However,there are some cases in which the communication bus has an asymmetricalimpedance drive. This results in an asymmetrical noise margin betweenthe high and low logic states. This is problematic when glitches occur,since the tri-state bus can be driven to an incorrect logic level when aconventional bus keeper circuit is used.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the various embodiments described herein,and to show more clearly how these various embodiments may be carriedinto effect, reference will be made, by way of example, to theaccompanying drawings which show at least one example embodiment, and inwhich:

FIG. 1 is a block diagram of an example embodiment of an asymmetricalbus keeper circuit with an asymmetrical inverter stage;

FIG. 2 is a schematic diagram of an example embodiment of anasymmetrical bus keeper circuit with an asymmetrical inverter stage forproviding asymmetrical drive to the high logic level;

FIG. 3 a is an example signal diagram in which the top panel shows thevoltage on a bus line and the bottom panel shows the output current of aconventional symmetrical bus keeper circuit;

FIG. 3 b is an example signal diagram in which the top panel shows thevoltage on a bus line and the bottom panel shows the output current ofan asymmetrical bus keeper circuit that provides asymmetrical drive tothe high logic level;

FIG. 4 is a schematic diagram of another example embodiment of anasymmetrical bus keeper circuit with an asymmetrical inverter stage forproviding asymmetrical drive to the low logic level;

FIG. 5 is a schematic diagram of an example embodiment of anasymmetrical bus keeper circuit that uses Bipolar Junction Transistors(BJT) with an asymmetrical inverter stage for providing asymmetricaldrive to the high logic level; and

FIG. 6 is a schematic diagram of another example embodiment of anasymmetrical bus keeper circuit that uses BJT transistors with anasymmetrical inverter stage for providing asymmetrical drive to the lowlogic level.

DETAILED DESCRIPTION OF THE EMBODIMENTS

It will be appreciated that for simplicity and clarity of illustration,where considered appropriate, reference numerals may be repeated amongthe figures to indicate corresponding or analogous elements. Inaddition, numerous specific details are set forth in order to provide athorough understanding of the embodiments described herein. However, itwill be understood by those of ordinary skill in the art that theembodiments described herein may be practiced without these specificdetails. In other instances, well-known methods, procedures andcomponents have not been described in detail so as not to obscure theembodiments described herein. Also, the description is not to beconsidered as limiting the scope of the embodiments described herein.

Referring now to FIG. 1, shown therein is a block diagram of an exampleembodiment of an asymmetrical bus keeper circuit 10. The asymmetricalbus keeper circuit 10 comprises a first inverter stage 14, anasymmetrical inverter stage 16 and a feedback stage 18. The firstinverter stage 14 has an input node that is connected to theinput/output node (Vi/o) of the circuit 10. The input node of theasymmetrical inverter stage 16 is connected to the output node of thefirst inverter stage 14. The input node of the feedback stage 18 isconnected to the output node of the asymmetrical inverter stage 16 andthe output node of the feedback stage 18 is connected to the input nodeof the first inverter stage 14 and the input/output node (Vi/o) of thecircuit 10. The input/output node (Vi/o) of the asymmetrical bus keepercircuit 10 is connected to an asymmetrical bus line 12 to receive aninput voltage and provide an output voltage. In other uses, the input ofthe asymmetrical bus keeper circuit 10 can be connected to a circuitboard input.

The first inverter stage 14 comprises transistors that are balanced withone another since they have similar physical sizes and can providesimilar current drive (i.e. they have a similar Beta). However, theasymmetrical inverter stage 16 is implemented such that it provides anasymmetrical drive towards one of the logic levels, which is the lastlogic level that was on the bus line 12. The feedback stage 18 isimplemented such that the drive is weak so that it can be over-ridden byother devices. Therefore, the asymmetrical bus keeper circuit 10provides weak drive in one logic state and no drive in the other logicstate. This differs from a conventional bus keeper circuit whichprovides symmetrical weak drive in both logic states on the bus line 12.This allows the asymmetrical bus keeper circuit 10 to be used with anasymmetrical bus line and to be able to deal with glitches on theasymmetrical bus line, as will be shown in more detail below withregards to FIGS. 3 a-3 b. An asymmetrical bus line is a bus line whichhas an asymmetrical bus protocol, which is similar to a floating busprotocol in which more current needs to be driven in one direction (i.e.logic level) than the other.

The implementation of the asymmetrical inverter stage 16 depends onwhich logic state the asymmetrical bus drives towards. For example, inthe case in which the asymmetrical bus drives to the low logic levelwith low impedance, but floats after a high logic level is achieved, theasymmetrical inverter stage 16 is implemented to provide a weak pull-upwhen the voltage on the asymmetrical bus line is high, but no pull-upwhen the voltage on the asymmetrical bus line is low. Conversely, forthe case in which the asymmetrical bus line drives to the high logiclevel with low impedance, but floats after a low logic level isachieved, the asymmetrical inverter stage 16 is implemented to provide aweak pull-down when the voltage on the asymmetrical bus line is low, butno pull-down when the voltage on the asymmetrical bus line is high. Inthe first case, the pull-up impedance of the bus line 12 can be improvedfrom using a very high resistance to just a few thousand ohms in thehigh state without causing any current draw in the low state. Likewise,in the second case, the pull-down impedance of the bus line 12 can beimproved from using a very high resistance to just a few thousand ohmsin the low state without causing any current draw in the high state.While pull-up or pull-down resistors can provide a similar function asthe asymmetrical bus keeper circuit 10, these resistors do not turn offin the opposite drive state and therefore waste power in the oppositedrive state. Accordingly, the power dissipation associated with theasymmetrical bus keeper circuit 10 is much lower than the powerdissipation associated with a conventional symmetrical bus keepercircuit or with bus keeper circuits that use either a pull-up resistoror a pull-down resistor. The reduction in power dissipation in theasymmetrical bus keeper circuits described herein is very importantespecially in battery-driven devices such as hand-held devices (i.e.,devices sized and shaped to be held or carried in a human hand), whichmay include mobile phones, smart phones, laptops, notepads, tabletcomputers and the like.

Another advantage of using the asymmetrical bus keeper circuit 10instead of a conventional symmetrical bus keeper circuit for anasymmetrical bus line is that bus impairments such as ringing orcrosstalk, which occur near the logic threshold of the input of theasymmetrical bus keeper circuit 10, will not force the circuit into theincorrect logic state. Therefore, the asymmetrical bus keeper circuit 10has the ability to restore the bus to the proper logic state on theopposite side of the ring cycle. This is described in further detailwith regards to FIGS. 3 a-3 b.

Referring now to FIG. 2, shown therein is a schematic diagram of anexample embodiment of an asymmetrical bus keeper circuit 20 with a firstinverter stage 24, an asymmetrical inverter stage 26 and a feedbackstage 28. The asymmetrical inverter stage 26 can provide asymmetricaldrive to the high logic level and so the asymmetrical bus keeper circuit20 can be considered to be a pull-up only bus keeper.

The first inverter stage 24 comprises a p-channel transistor 24 a and ann-channel transistor 24 b in an inverter configuration in which thesource of the p-channel transistor 24 a is connected to the positivevoltage supply level Vdd, the gate of the p-channel transistor 24 a isconnected to the gate of the n-channel transistor 24 b, the drain of thep-channel transistor 24 a is connected to the drain of the n-channeltransistor 24 b and the source of the n-channel transistor 24 b isconnected to ground. The current drive (i.e. size) of the p-channel andn-channel transistors 24 a and 24 b are similar so that the firstinverter stage 14 operates in a balanced manner. The transistors 24 aand 24 b are CMOS transistors.

The asymmetrical inverter stage 26 is implemented by using an open drainkeeper driver, which is in the form of a single p-channel transistor 26a in this case. The gate of the p-channel transistor 26 a is connectedto the input node of the asymmetrical inverter stage 26, the source ofthe p-channel transistor 26 a is connected to the positive voltagesupply Vdd and the drain of the p-channel transistor 26 a is connectedto the output node of the asymmetrical inverter stage 26 which isconnected to the input node of the feedback stage 28. The drive of thetransistor 26 a is selected to be similar to the drives of thetransistors 24 a and 24 b.

In an alternative embodiment, the asymmetrical inverter stage 26 canhave a p-channel transistor and an n-channel transistor connected in aninverter configuration, similar to transistors 24 a and 24 b, but withasymmetrical drives. In this case, the upper p-channel transistor in theasymmetrical inverter stage would have a much larger drive (i.e. Beta)than the lower n-channel transistor and the drive of the n-channeltransistor would be selected such that it essentially acts as an opencircuit. For example, if the output drive of the upper transistor isabout 2 mA, then the output drive of the lower transistor can beselected to be about 1 to 3 μA (i.e. three orders of magnitudes lower).However, in general, the output drive of the lower transistor can beabout 1 to 4 orders of magnitude smaller than the output drive of theupper transistor.

The feedback stage 28 comprises a resistor having a resistance of 1kilo-ohm to several kilo-ohms to limit the amount of output current ofthe asymmetrical bus keeper circuit 20 and ensure that it provides weakdrive. In alternative embodiments, which are applicable to all of thecircuit schematics shown herein, the feedback stage 28 can also includea capacitor, with a small capacitance, that is placed in parallel withthe resistor in order to speed up the feedback.

The asymmetrical bus keeper circuit 10 is useful in situations where abus line can be driven high or low but it is sometimes left at a highimpedance logic level for a short time. When the bus line 12 is beingpulled low, no drive is provided as the transistor 26 a turns off andeffectively acts as an open circuit. Alternatively, when the bus line 12is being pulled high, the transistor 26 a turns on and provides a weakdrive through the feedback stage 28 and a weak pull-up to maintain thehigh logic level on the bus line 12. Therefore, the output of theasymmetrical bus keeper circuit 20 is a weak high logic level when thebus line 12 is being driven high and when the bus line 12 is beingdriven low, the output of the asymmetrical bus keeper circuit 20 is off.Whenever the voltage on the bus line 12 transitions to the high logicstate or the low logic state, there is a momentary transient where theasymmetrical bus keeper circuit 20 is active and conducting current. Inthe rising voltage level case, the current provided by the asymmetricalbus keeper circuit 20 helps the signal on the bus line 12 rise to thehigh logic state. In the falling voltage level case, the currentprovided by the asymmetrical bus keeper circuit 20 opposes the fall involtage until the signal on the bus line 12 transitions almost to zeroVolts. Accordingly, in both the high level and low logic level states onthe bus line 12, the asymmetrical bus keeper 20 draws very little or nocurrent. Furthermore, by using a relatively large resistance, asdescribed above, in the feedback stage 28, power consumption isminimized when weak drive is provided by the asymmetrical bus keepercircuit 10. In addition, since a weak drive is provided by theasymmetrical bus keeper circuit 20, contention is avoided since theasymmetrical bus keeper circuit 20 will not be able to pull the bus line10 high when another device connected to the bus line 10 is trying topull the bus line 10 low.

The situation in which the voltage on the bus line 12 is at anintermediate voltage in the linear region of a CMOS transistor can bedangerous for CMOS circuits since it will cause both the n-channel andp-channel transistors to turn on (known as “mutual conduction” or “shootthrough”) and create a short circuit between the positive voltage supplyand ground. This short circuit could cause a very large current to flowwhich would heat the transistors, waste power and possibly damage thetransistors depending on how long the voltage on the bus line 12 is inthe linear region of the input transistors. However, this situation isnot problematic for the asymmetrical bus keeper circuit 20. While boththe transistors 24 a and 24 b would be slightly on in this condition,the result is that the input voltage to the transistor 26 a is also atan intermediate voltage so the transistor 26 a would be slightly on andthe output of the asymmetrical inverter stage 16 would be weakly drivento the power rail (i.e. positive supply voltage level). Therefore, theasymmetrical bus keeper circuit 20 would quickly return the bus line 12to the high logic level state without the need for a pull-up resistorand therefore also avoid the wasted current and power dissipation thatoccurs at the pull-up resistor when the asymmetrical bus keeper circuit20 is in the low logic state.

Referring now to FIG. 3 a, shown therein is an example signal diagram inwhich the top panel shows the voltage on a bus line and the bottom panelshows the output voltage of a conventional symmetrical bus keepercircuit. A voltage glitch occurs on the bus line at 300 nanosecondswhich pulls the voltage on the bus line low which results in aconduction of current through the conventional symmetrical bus keepercircuit since the p-channel transistor in the first inverter stage turnson which turns on the n-channel transistor in the second inverter stage.The result is that the n-channel transistor in the second inverter stageweakly holds the output voltage of the conventional symmetrical buskeeper circuit low as can be seen in the top panel of FIG. 3 a, which isincorrect and undesirable.

Referring now to FIG. 3 b, shown therein is an example signal diagram inwhich the top panel is similar to that of FIG. 3 a and the bottom panelshows the output current of the asymmetrical bus keeper circuit 20. Ascan be seen, whenever there is a transition from a low logic level to ahigh logic level and vice versa on the bus line 12, the asymmetrical buskeeper circuit 20 conducts current for an extremely short period oftime. Furthermore, whenever there is a short duration weak negativetransient which pulls the voltage on the bus line 12 slightly below theswitching threshold of the transistors in the asymmetrical bus keepercircuit 20, the asymmetrical bus keeper circuit 20 stops pulling up thevoltage weakly during the transient but does not flip the logic state ofthe bus line 12 as shown in FIG. 3 b. Once the transient ends, if thefinal state on the bus line 12 is still high impedance but just abovethe logic high threshold of the transistors in the asymmetrical buskeeper circuit 20, the asymmetrical bus keeper circuit 20 will onceagain pull the voltage on the bus line 12 up weakly, which is correctand desirable. This is in contrast to the conventional symmetrical buskeeper circuit which flipped the state on the bus line 12 to the logiclow level.

Referring now to FIG. 4, shown therein is a schematic diagram of anotherexample embodiment of an asymmetrical bus keeper circuit 40 with anasymmetrical inverter stage 46 for providing asymmetrical drive to thelow logic level for the bus line 12. The asymmetrical inverter stage 46is in an open-drain configuration. The asymmetrical bus keeper circuit40 provides a weak pull-down when the bus line 12 is at the logic lowlevel and provides no pull-up when the bus line 12 is at the logic highlevel. Accordingly, the asymmetrical bus keeper circuit 40 operates inan opposite manner as the asymmetrical bus keeper circuit 20 withrespect to drive and logic level but it provides all of the samebenefits as the asymmetrical bus keeper circuit 20.

The asymmetrical inverter stage 46 is implemented by using an open drainkeeper driver, which is in the form of a single n-channel transistor 46b in this case. The gate of the n-channel transistor 46 b is connectedto the input node of the asymmetrical inverter stage 46, the source ofthe n-channel transistor 46 b is connected to ground and the drain ofthe n-channel transistor 46 b is connected to the output node of theasymmetrical inverter stage 46. The output node of the asymmetricalinverter stage 46 is connected to the input node of the feedback stage28. The drive of the transistor 46 b is selected to be similar to thedrives of the transistors 24 a and 24 b.

In an alternative embodiment, the asymmetrical inverter stage 46 canhave a p-channel transistor and an n-channel transistor connected in aninverter configuration, similar to transistors 24 a and 24 b, but withasymmetrical drives. In this case, the upper p-channel transistor in theasymmetrical inverter stage would have a much smaller drive (i.e. Beta)than the lower n-channel transistor and the drive of the p-channeltransistor would be selected such that it essentially acts as an opencircuit. For example, if the output drive of the lower transistor isabout 2 mA, then the output drive of the upper transistor can beselected to be about 1 to 3 μA (i.e. three orders of magnitudes lower).However, in general, the output drive of the lower transistor can beabout 1 to 4 orders of magnitude larger than the output drive of theupper transistor.

For the situation in which the voltage on the bus line 12 is at anintermediate voltage in the linear region of a CMOS transistor, both thetransistors 24 a and 24 b would be slightly on in this condition, butthe input voltage to the transistor 46 b is also at an intermediatevoltage so the transistor 46 b would be slightly on and the output ofthe asymmetrical inverter stage 46 would be weakly driven to ground.Therefore, the asymmetrical bus keeper circuit 40 would quickly returnthe bus line 12 to the low logic level state without the need for apull-down resistor and therefore also avoid the wasted current and powerdissipation that occurs at the pull-down resistor when the asymmetricalbus keeper circuit 20 is in the high logic state.

It should be understood that the signal diagrams of FIG. 3 b and theproperties of the asymmetrical bus keeper circuits 10 and 20, such as indealing with glitches for example, also apply to the asymmetrical buskeeper circuit 40. However, the signal diagram for the asymmetrical buskeeper circuit 40 would be the opposite of what is shown in FIG. 3 b.Accordingly, when the bus line 12 is at a low logic level and a shortduration weak positive transient (i.e. glitch) occurs that increases thevoltage level slightly above the switching threshold of the transistorsin a bus keeper circuit, a conventional symmetrical bus keeper circuitwould flip the logic state and hold the bus line 12 at a high logiclevel. However, the asymmetrical bus keeper circuit 40 would stoppulling down the voltage weakly during the transient but would not flipthe logic state of the bus line 12. Once the transient ends, if thefinal state on the bus line 12 is still a low logic level, theasymmetrical bus keeper circuit 40 will once again pull the voltage onthe bus line 12 low weakly.

The asymmetrical bus keeper concept can also be extended to BipolarJunction Transistor (BJT) based bus keeper circuits. There are a largenumber of BJT inverter structures that currently exist which differ inspeed, power, input impedance, output level, supply voltage and thelike, as well as logic family such as Resistor Transistor Logic (RTL),Diode Transistor Logic (DTL), Emitter-Coupled Logic (ECL) and the like.While there are a large number of BJT inverter structures, the conceptof asymmetrical drive can be extended to these different BJT inverterstructures to form asymmetrical BJT bus keeper circuits by modifying thecircuit topology in the second inverter stage such that it providesasymmetrical drive to either one of the logic levels. Generally, all ofthese different BJT inverter topologies have a totem pole outputstructure and asymmetrical drive can be obtained by modifying the totempole output structure. This is shown in FIG. 5 for an example embodimentof an asymmetrical BJT bus keeper circuit 50 that uses a particular BJTinverter structure. The asymmetrical bus keeper circuit 50 includes asymmetrical inverter stage 54 and an asymmetrical inverter stage 56 forproviding asymmetrical drive to the high logic level. Another exampleembodiment of an asymmetrical BJT bus keeper circuit 60 is shown in FIG.6. The asymmetrical BJT bus keeper circuit 60 includes the symmetricalinverter stage 54 and an asymmetrical inverter stage 66 that providesasymmetrical drive to the low logic level. In both of these exampleembodiments, the totem pole output structure of the symmetrical inverterstage 54 includes a resistor R4, a transistor Q3, a diode D2 and anothertransistor Q4 connected in series. However, an asymmetrical drive can beobtained by removing one of the two transistors in the totem pole outputstructure of the asymmetrical inverter stages 56 and 66. For theasymmetrical inverter stage 56, the upper portion of the totem poleoutput structure has a transistor Q7 but not the lower portion of thetotem pole output structure thereby providing asymmetrical drive to thehigh logic level. For the asymmetrical inverter stage 66, the lowerportion of the totem pole output structure has a transistor Q8 but notthe upper portion of the totem pole output structure thereby providingasymmetrical drive to the low logic level.

In both of these example embodiments, the resistances of the resistorsand the size of the transistors in the symmetrical inverter stage 54 areselected depending on the magnitude of the supply voltage and thecorresponding required drive current as is known by those skilled in theart. The resistances of the resistors and the sizes of the transistorsin the asymmetrical inverter stages 56 and 66 are then selected in acorresponding fashion. For instance, the BJT transistors all havegenerally similar current drive. The advantages and operation describedfor the asymmetrical bus keeper circuit 20 generally applies to theasymmetrical BJT bus keeper circuit 50 while the advantages andoperation described for the asymmetrical bus keeper circuit 40 generallyapplies to the asymmetrical BJT bus keeper circuit 60.

In alternative embodiments, the asymmetrical inverter stages 56 and 66can have a similar circuit topology as the symmetrical inverter stage54, however, the drive of the transistors in the upper and lower portionof the totem pole output structure are substantially different from oneanother to provide asymmetrical drive. In this alternative, theasymmetrical inverter stage 56 includes the resistor R8, the transistorsQ7 and Q8 and the diode D4. The base of the transistor Q8 is connectedto the emitter of the transistor Q6, the collector of the transistor Q8is connected to the cathode of the diode D4, and the emitter of thetransistor Q8 is connected to ground. The collector of the transistor Q8provides an output node for the asymmetrical inverter stage which isconnected to the feedback stage 28. In order to provide similarfunctionality as the asymmetrical BJT bus keeper circuit 50, the driveof the transistor Q7, in the upper portion of the totem pole outputstructure, is about 3 orders of magnitude greater than the drive of thetransistor Q8, although in general the drive of the transistor Q7 can bein the range of 1 to 4 orders of magnitude greater than the drive of thetransistor Q8. Alternatively, in order to provide similar functionalityas the asymmetrical BJT bus keeper circuit 60, the drive of thetransistor Q8, in the lower portion of the totem pole output structure,is about 3 orders of magnitude greater than the drive of the transistorQ7, although in general the drive of the transistor Q8 can be in therange of 1 to 4 orders of magnitude greater than the drive of thetransistor Q7.

In an alternative, the CMOS and BJT asymmetrical bus keeper circuitsdiscussed herein that provide a weak pull-up can be modified by adding alarge resistor, on the order of a Mega-ohm or so, between the input nodeof the asymmetrical bus keeper circuit and the positive supply voltageVdd or Vcc as the case may be. This resistor can be used to preventmetastability, which may occur in some cases depending on the nature ofthe bus (i.e. whether it is asynchronous rather than synchronous and/orwhether more than one independent clock system is used). Metastabilityoccurs when an intermediate (i.e. not high but not low) voltage isapplied to the input/output node of the asymmetrical bus keeper circuitin which case the voltage at the input/output node may take anindeterminate amount of time to settle to the desired logic value.

In another alternative, the CMOS and BJT asymmetrical bus keepercircuits discussed herein that provide a weak pull-down can be modifiedby adding a large resistor, on the order of a Mega-ohm or so, betweenthe input/output node of the asymmetrical bus keeper circuit and groundto prevent metastability as was just discussed.

In the CMOS and BJT asymmetrical bus keeper circuits discussed herein,the asymmetrical feedback will always eventually result in saturation tothe high or low logic state level, but in some cases this process mighttake a non-trivial amount of time. During this time, at the intermediatevoltage on the input/output node, the first CMOS inverter stage willwaste power while in the BJT cases some power, although not as much asin the CMOS case, will be wasted at the intermediate input voltage.Accordingly, in another alternative, for some cases depending on how theasymmetrical bus keeper circuits are used, it can be beneficial tomodify the CMOS and BJT asymmetrical bus keeper circuits discussedherein that provide asymmetrical drive to the high logic level by addinga very large resistor between the output node of the asymmetric inverterstage and the positive voltage supply Vdd or Vcc, as the case may be, toprevent this ambiguous floating mid-voltage logic state in theasymmetrical bus keeper circuits. This pull-up resistor can have aresistance on the order of 100 kilo-ohms to 1 Mega-ohm. The pull-upresistor can shorten the time required to get out of the linear regionby several orders of magnitude. In a likewise fashion, for some casesdepending on how the asymmetrical bus keeper circuits are used, it canbe beneficial to modify the CMOS and BJT asymmetrical bus keepercircuits discussed herein that provide asymmetrical drive to the logiclow level by adding a very large resistor between the output node of theasymmetrical inverter stage and ground to prevent the ambiguous floatingmid-voltage logic state in the asymmetrical bus keeper circuits. Thisresistor can have a resistance on the order of 100 kilo-ohms to 1Mega-ohm.

The asymmetrical bus keeper circuits described herein can be used withan asymmetrical bus which requires an asymmetrical termination. Theasymmetrical bus keeper circuits described herein can also be used toconnect several electronic devices together, to connect an electronicdevice to a peripheral device or an external device, or to connectseveral Integrated Circuits (ICs) together.

The asymmetrical bus keeper circuits described herein can also be usedwithin an IC as well as in dynamic logic circuits. Dynamic logiccircuits set a node to the high or low logic level and then let the nodefloat until either another device changes the logic state or the logicstate of the node is read out. In particular, in dynamic logic, only onedevice may drive the bus to one logic state (for example a high logiclevel) but all other devices attached to the node or bus may only everdrive the bus to the opposite logic state (i.e. the low logic level). Indynamic logic, an initial logic high state may be imposed on a node orbus by one device and then one or more other devices may subsequentlyeither not drive the node at all or drive the node or bus to the logiclow level at a later time in a wired “AND” configuration. If none of theother devices drive the node low then the bus remains in the logic highstate but is vulnerable to noise due to no active drive to the highstate. If any device were to drive it low, then it would not bevulnerable to noise since the logic low driver would be at lowimpedance. Thus, the bus drive is asymmetrical. This results in anasymmetrical noise margin between the high and low logic states. Forinstance, in the dynamic logic example, the high logic state noisemargin may be as little as 0 Volts, whereas the low logic state noisemargin is the difference between the logic low output level and theinput threshold for the logic high output level.

In one aspect, at least one example embodiment described herein, thereis provided an asymmetrical bus keeper circuit that comprises a firstinverter stage having an input node and an output node; an asymmetricalinverter stage having an input node and an output node; the input nodeof the asymmetrical inverter stage being connected to the output node ofthe first inverter stage; and a feedback stage with an input node and anoutput node, the input node of the feedback stage being connected to theoutput node of the asymmetrical inverter stage and the output node ofthe feedback stage being connected to the input node of the firstinverter stage. The asymmetrical stage provides an asymmetrical drivetowards one logic level.

In at least some embodiments, the feedback stage comprises a resistor.

In at least some embodiments, the asymmetrical inverter stage isconfigured to provide the asymmetrical drive to a high logic level andthe asymmetrical inverter stage comprises a single p-channel transistorwherein a gate of the p-channel transistor is connected to the inputnode of the asymmetrical inverter stage, a source of the p-channeltransistor is connected to a positive voltage supply level, and a drainof the p-channel transistor is connected to the output node of theasymmetrical inverter stage.

In at least some embodiments, the asymmetrical inverter stage isconfigured to provide the asymmetrical drive to a high logic level andthe asymmetrical inverter stage comprises a p-channel transistor havinga first drive and an n-channel transistor having a second drive, thep-channel and n-channel transistors being oriented in an inverterconfiguration, wherein the first drive is at one order of magnitudelarger than the second drive.

In at least some embodiments, the asymmetrical inverter stage isconfigured to provide the asymmetrical drive to a low logic level andthe asymmetrical inverter stage comprises a single n-channel transistorwherein a gate of the n-channel transistor is connected to the inputnode of the asymmetrical inverter stage, a source of the n-channeltransistor is connected to ground, and a drain of the n-channeltransistor is connected to the output node of the asymmetrical inverterstage.

In at least some embodiments, the asymmetrical inverter stage isconfigured to provide the asymmetrical drive to a low logic level andthe asymmetrical inverter stage comprises a p-channel transistor havinga first drive and an n-channel transistor having a second drive, thep-channel and n-channel transistors being oriented in an inverterconfiguration, wherein the second drive is at least one order ofmagnitude larger than the first drive.

In at least some embodiments, the asymmetrical inverter stage isconfigured to provide the asymmetrical drive to a high logic level andthe asymmetrical bus keeper circuit further comprises a pull-up resistorconnected between the input node of the first inverter stage and apositive voltage supply level.

In at least some embodiments, the asymmetrical inverter stage isconfigured to provide the asymmetrical drive to a low logic level andthe asymmetrical bus keeper circuit further comprises a pull-downresistor connected between the input node of the first inverter stageand ground.

In at least some embodiments, the asymmetrical inverter stage isconfigured to provide the asymmetrical drive to a high logic level andthe asymmetrical bus keeper circuit further comprises a resistorconnected between the output node of the asymmetrical inverter stage andthe positive voltage supply level.

In at least some embodiments, the asymmetrical inverter stage isconfigured to provide the asymmetrical drive to a low logic level andthe asymmetrical bus keeper circuit further comprises a resistorconnected between the output node of the asymmetrical inverter stage andground.

In at least some embodiments, the symmetrical and asymmetrical inverterstages comprise bipolar junction transistors and the asymmetricalinverter stage comprises a totem pole output structure with only anupper portion having a bipolar junction transistor to provide theasymmetrical drive to a high logic level.

In at least some embodiments, the symmetrical and asymmetrical inverterstages comprise bipolar junction transistors and the asymmetricalinverter stage comprises a totem pole output structure with only a lowerportion having a bipolar junction transistor to provide the asymmetricaldrive to a low logic level.

In at least some embodiments, the symmetrical and asymmetrical inverterstages comprise bipolar junction transistors and the asymmetricalinverter stage comprises a totem pole output structure with an upperportion having a bipolar junction transistor with a first drive and alower portion having a bipolar junction transistor with a second drive,wherein the first drive is at least one order of magnitude larger thanthe second drive to provide the asymmetrical drive to a high logiclevel.

In at least some embodiments, the symmetrical and asymmetrical inverterstages comprise bipolar junction transistors and the asymmetricalinverter stage comprises a totem pole output structure with an upperportion having a bipolar junction transistor with a first drive and alower portion having a bipolar junction transistor with a second drive,wherein the second drive is at least one order of magnitude larger thanthe first drive to provide the asymmetrical drive to a low logic level.

It should be understood that various modifications can be made to theembodiments described and illustrated herein, without departing from theembodiments, the general scope of which is defined in the appendedclaims.

The invention claimed is:
 1. An asymmetrical bus keeper circuitcomprising: a first inverter stage having an input node and an outputnode; an asymmetrical inverter stage having an input node and an outputnode; the input node of the asymmetrical inverter stage being connectedto the output node of the first inverter stage; and a feedback stagewith an input node and an output node, the input node of the feedbackstage being connected to the output node of the asymmetrical inverterstage and the output node of the feedback stage being connected to theinput node of the first inverter stage, wherein the asymmetricalinverter stage provides an asymmetrical drive towards one logic leveland the feedback stage comprises a resistor.
 2. The asymmetrical buskeeper circuit of claim 1, wherein the asymmetrical inverter stage isconfigured to provide the asymmetrical drive to a high logic level andthe asymmetrical inverter stage comprises a single p-channel transistorwherein a gate of the p-channel transistor is connected to the inputnode of the asymmetrical inverter stage, a source of the p-channeltransistor is connected to a positive voltage supply level, and a drainof the p-channel transistor is connected to the output node of theasymmetrical inverter stage.
 3. The asymmetrical bus keeper circuit ofclaim 1, wherein the asymmetrical inverter stage is configured toprovide the asymmetrical drive to a high logic level and theasymmetrical inverter stage comprises a p-channel transistor having afirst drive and an n-channel transistor having a second drive, thep-channel and n-channel transistors being oriented in an inverterconfiguration, wherein the first drive is at least one order ofmagnitude larger than the second drive.
 4. The asymmetrical bus keepercircuit of claim 1, wherein the asymmetrical inverter stage isconfigured to provide the asymmetrical drive to a low logic level andthe asymmetrical inverter stage comprises a single n-channel transistorwherein a gate of the n-channel transistor is connected to the inputnode of the asymmetrical inverter stage, a source of the n-channeltransistor is connected to ground, and a drain of the n-channeltransistor is connected to the output node of the asymmetrical inverterstage.
 5. The asymmetrical bus keeper circuit of claim 1, wherein theasymmetrical inverter stage is configured to provide the asymmetricaldrive to a low logic level and the asymmetrical inverter stage comprisesa p-channel transistor having a first drive and an n-channel transistorhaving a second drive, the p-channel and n-channel transistors beingoriented in an inverter configuration, wherein the second drive is atleast one order of magnitude larger than the first drive.
 6. Theasymmetrical bus keeper circuit of claim 1, wherein the asymmetricalinverter stage is configured to provide the asymmetrical drive to a highlogic level and the asymmetrical bus keeper circuit further comprises apull-up resistor connected between the input node of the first inverterstage and a positive voltage supply level.
 7. The asymmetrical buskeeper circuit of claim 1, wherein the asymmetrical inverter stage isconfigured to provide the asymmetrical drive to a low logic level andthe asymmetrical bus keeper circuit further comprises a pull-downresistor connected between the input node of the first inverter stageand ground.
 8. The asymmetrical bus keeper circuit of claim 1, whereinthe asymmetrical inverter stage is configured to provide theasymmetrical drive to a high logic level and the asymmetrical bus keepercircuit further comprises a resistor connected between the output nodeof the asymmetrical inverter stage and the positive voltage supplylevel.
 9. The asymmetrical bus keeper circuit of claim 1, wherein theasymmetrical inverter stage is configured to provide the asymmetricaldrive to a low logic level and the asymmetrical bus keeper circuitfurther comprises a resistor connected between the output node of theasymmetrical inverter stage and ground.
 10. The asymmetrical bus keepercircuit of claim 1, wherein the symmetrical and asymmetrical inverterstages comprise bipolar junction transistors and the asymmetricalinverter stage comprises a totem pole output structure with only anupper portion having a bipolar junction transistor to provide theasymmetrical drive to a high logic level.
 11. The asymmetrical buskeeper circuit of claim 1, wherein the symmetrical and asymmetricalinverter stages comprise bipolar junction transistors and theasymmetrical inverter stage comprises a totem pole output structure withonly a lower portion having a bipolar junction transistor to provide theasymmetrical drive to a low logic level.
 12. The asymmetrical bus keepercircuit of claim 1, wherein the symmetrical and asymmetrical inverterstages comprise bipolar junction transistors and the asymmetricalinverter stage comprises a totem pole output structure with an upperportion having a bipolar junction transistor with a first drive and alower portion having a bipolar junction transistor with a second drive,wherein the first drive is at least one order of magnitude larger thanthe second drive to provide the asymmetrical drive to a high logiclevel.
 13. The asymmetrical bus keeper circuit of claim 1, wherein thesymmetrical and asymmetrical inverter stages comprise bipolar junctiontransistors and the asymmetrical inverter stage comprises a totem poleoutput structure with an upper portion having a bipolar junctiontransistor with a first drive and a lower portion having a bipolarjunction transistor with a second drive, wherein the second drive is atleast one order of magnitude larger than the first drive to provide theasymmetrical drive to a low logic level.
 14. The asymmetrical bus keepercircuit of claim 1, wherein the asymmetrical inverter stage isconfigured to provide the asymmetrical drive to a high logic level andthe asymmetrical inverter stage comprises a first transistor having afirst drive and an second transistor having a second drive, wherein thefirst drive is at least one order of magnitude larger than the seconddrive.
 15. An asymmetrical bus keeper circuit comprising: a firstinverter stage having an input node and an output node; an asymmetricalinverter stage having an input node, a single p-channel transistor andan output node, a gate of the p-channel transistor being connected tothe input node of the asymmetrical inverter stage, a source of thep-channel transistor being connected to a positive voltage supply level,a drain of the p-channel transistor being connected to the output nodeof the asymmetrical inverter stage and the input node of theasymmetrical inverter stage being connected to the output node of thefirst inverter stage; and a feedback stage with an input node and anoutput node, the input node of the feedback stage being connected to theoutput node of the asymmetrical inverter stage and the output node ofthe feedback stage being connected to the input node of the firstinverter stage, wherein the asymmetrical inverter stage is configured toprovide the asymmetrical drive to a high logic level.
 16. Theasymmetrical bus keeper circuit of claim 15, wherein the asymmetricalbus keeper circuit further comprises a pull-up resistor connectedbetween the input node of the first inverter stage and a positivevoltage supply level.
 17. The asymmetrical bus keeper circuit of claim15, wherein the asymmetrical bus keeper circuit further comprises aresistor connected between the output node of the asymmetrical inverterstage and the positive voltage supply level.
 18. An asymmetrical buskeeper circuit comprising: a first inverter stage having an input nodeand an output node; an asymmetrical inverter stage having an input node,a single n-channel transistor and an output node, a gate of then-channel transistor being connected to the input node of theasymmetrical inverter stage, a source of the n-channel transistor beingconnected to ground, a drain of the n-channel transistor being connectedto the output node of the asymmetrical inverter stage, and the inputnode of the asymmetrical inverter stage being connected to the outputnode of the first inverter stage; and a feedback stage with an inputnode and an output node, the input node of the feedback stage beingconnected to the output node of the asymmetrical inverter stage and theoutput node of the feedback stage being connected to the input node ofthe first inverter stage, wherein the asymmetrical inverter stage isconfigured to provide the asymmetrical drive to a low logic level. 19.The asymmetrical bus keeper circuit of claim 18, wherein theasymmetrical bus keeper circuit further comprises a pull-down resistorconnected between the input node of the first inverter stage and ground.20. The asymmetrical bus keeper circuit of claim 18, wherein theasymmetrical bus keeper circuit further comprises a resistor connectedbetween the output node of the asymmetrical inverter stage and ground.